1. Field of the Invention
The present invention relates generally to a semiconductor device having a bump electrode and manufacturing method thereof and, more particularly, to a semiconductor device having a bump electrode formed with high integration and high accuracy and a method enabling manufacturing of the same.
2. Description of the Background Art
A conventional semiconductor device having a bump electrode and a method of manufacturing the same will be described in the following with reference to FIGS. 10-22.
Referring to FIG. 10, a number of electronic elements (not shown) and a plurality of interconnection layers (not shown) are formed on a semiconductor substrate 41. An integrated circuit (not shown) is constituted by electrically connecting these electronic elements with interconnection layer. A plurality of bump electrodes 63 are formed on semiconductor substrate 41. Bump electrodes 63 are electrically connected to the integrated circuit formed on semiconductor substrate 41. A circuit board (not shown) and the integrated circuit formed on semiconductor substrate 41 are electrically connected through bump electrodes 63. Leads 67 in the form of a plate is supported by a film 65 on semiconductor substrate 41, one end of each lead 67 electrically connected to bump electrode 63.
Referring to FIG. 11, pad electrodes 43 are formed with a distance posed therebetween on semiconductor substrate 41. On semiconductor substrate 41, a insulating layer 45 is formed and a opening 45a exposing a part of a surface of pad electrode 43 is formed on insulating layer 45.
An under-bump layer 47 is formed on insulating layer 45. Under-bump layer 47 is electrically connected to pad electrode 43 through a opening 45a. A bump electrode 63 is formed on under-bump layer 47, and bump electrode 63 and under-bump layer 47 are electrically connected. Lead 67 which is electrically connected to bump electrode 63 is formed on bump electrode 63.
There is a method of directly bonding wires made of gold, aluminum or the like to pad electrode 43 without using under-bump layer 47 and bump electrode 63. However, this method has the following defects. When it is necessary to form a number of pad electrodes 43 on semiconductor substrate 41, the distance between two of pad electrodes 43 becomes short. When the distance between pad electrodes 43 is short and the wire is bonded deviated from a desired pattern, the wire may be short circuited with an adjacent wire. Therefore, when the distance between two pad electrodes 43 is short, a structure having lead 67 in the form of a plate bonded to pad electrode 43 should be used. However, it is difficult to directly bond lead 67 to pad electrode 43 because pad electrode 43 is positioned in a recess. Therefore, bump electrode 63 which is electrically connected to pad electrode 43 is formed, and on bump electrode 63, lead 67 in the form of a plate is bonded.
A method of manufacturing a semiconductor device having a structure shown in FIG. 11 will be described.
As shown in FIG. 12, pad electrode 43 formed of AlCu alloy or the like, for example, is formed on semiconductor substrate 41. Pad electrode 43 is electrically connected to an integrated circuit (not shown) which is formed on semiconductor substrate 41. A insulating layer 45 of SiN, for example, is formed to cover pad electrode 43 on semiconductor substrate 41 by using CVD (Chemical Vapor Deposition) method, for example. A opening 45a exposing pad electrode 43 is formed in insulating layer 45 using photolithography and etching technology.
An under-bump layer 47 is formed on insulating layer 45 using sputtering, for example. Under-bump layer 47 has a two-layer structure, the lower layer of which is of Ti-W and the upper layer of which is of Au, for example. Under-bump layer 47 is electrically connected to pad electrode 43 through opening 45a.
As shown in FIG. 13, a photoresist 49 of 20-40 .mu.m in thickness is formed on under-bump layer 47 by spin coating a resist solution (BMR S-1000 manufactured by Tokyo Ohka Kogyo Co. Ltd., for example) of the viscosity of several hundreds to one thousand and several hundreds CPS at the rotation rate of several hundreds rpm. Then a mask 51 is aligned on photoresist 49. Mask 51 is made by forming a mask pattern 55 on a glass substrate 53. Mask 51 is in close contact tight with photoresist 49.
As shown in FIG. 14, by irradiating photoresist 49 with a light 59 through mask 51, photoresist 49 is selectively exposed. Light 59 does not reach a portion of photoresist 49 which is positioned just under mask pattern 55.
As shown in FIG. 15, a portion irradiated with light 59 is indicated by 49a and a portion which is not irradiated is indicated by 49b in photoresist 49. Molecular polymerization reaction in the photoresist (shown as X in the figures) occurs in a portion 49a which is irradiated with light 59.
FIG. 16 shows a state after a developing process of photoresist 49 is carried out. In photoresist 49, a portion 49a which is irradiated with light is left undissolved in a developer.
As shown in FIG. 17, a bump electrode 63 is formed using photoresist 49a as a mask by gold plating method using under-bump layer 47 as an electrode.
As shown in FIG. 18, photoresist 49a is removed. Then, under-bump layer 47 is selectively removed using bump electrode 63 as a mask.
By bonding lead 67 on bump electrode 63, manufacturing process of the structure shown in FIG. 11 is completed.
Method of exposing a photoresist includes contact exposing method, proximity exposing method and the like. In the contact exposing method, exposure is effected with a mask in close contact with a photoresist. When the contact between the mask and the photoresist is perfect, an exposure of pattern with a high resolution can be attained with small bad influence caused by diffraction of light. A pattern with a high resolution is, in other words, a fine-pitch pattern. In contact exposing method, using adhesion preventing solution is required for preventing a mask from adhering to the photoresist.
In the proximity exposing method, exposure is carried out with a gap of several tens .mu.m to several hundreds .mu.m between the mask and the photoresist. By this method, the defects of contact exposing method can be eliminated. However, the resolution is lower than that of contact exposing method because of the bad influence such as diffraction of light phenomenon.
The resolution of photoresist 49 (see FIG. 13) which is conventionally used in forming a bump electrode is, provided that the thickness thereof is 30 .mu.m, about 10 .mu.m when the contact exposing method is used and about 50 .mu.m when the proximity exposing method is used. As shown in FIG. 13, the contact exposing method is conventionally used, therefore such resolution is sufficient. However, it is desired to use proximity exposing method to avoid the above-mentioned defects of the contact exposing method and to obtain a resolution of 30 .mu.m or less when the thickness of the photoresist is about 30 .mu.m.
A in FIG. 19 shows an illuminance of light passing between mask patterns 55. Illuminance of the light near the edges of mask patterns 55 is low. Therefore, lower portion of photoresist 49 just under the vicinity of the edges of mask patterns 55 is not sufficiently irradiated with light. Therefore, in fact, the configuration of 49a, which is a portion of photoresist 49 irradiated with light, is as shown in FIG. 20. That is, the lower, the smaller the distance between right and left side faces 49c, 49c of photoresist 49a.
When the distance between right and left side faces 49c, 49c much reduces at the lower portion, right and left side faces 49c, 49c are connected and when development is carried out, photoresist 49a, which is to be left, is removed as shown in FIG. 21. Therefore, when such photoresist 49a is used as a mask to form a bump electrode, two adjacent bump electrodes are formed connected to each other.
When the sensitivity of photoresist 49 is increased, molecular polymerization reaction occurs in photoresist 49 even though illuminance is not enough. Therefore, it may be that the distance between side faces 49c, 49c is not much decreased if a high sensitivity of photoresist 49 is attained. However., in fact, the distance between side faces 49c, 49c becomes narrower as it goes lower as shown in FIG. 22 even though the high sensitivity of photoresist 49 is attained. The reason is that polymerization reaction proceeds rapidly at the upper portion of photoresist 49 and light hardly reaches the lower portion of photoresist 49 when a high sensitivity of photoresist 49 is attained.